2010 Dirac medal

For his contribution to our understanding of how galaxies are constituted, how they work and how they were formed.

In 1976 James Binney showed that low rotation speeds measured in elliptical galaxies arise naturally if these objects form by anisotropic collapse. In 1977 he derived a characteristic lengthscale and mass for galaxy formation above which infalling gas heats effectively. In 1993 he and a student pioneered the view that “cooling flows” in X-ray emitting clusters of galaxies neither radiate nor cool because they are episodically heated by black holes in “radio mode”. In 2004 he was the first to argue that these results together explain the absence of extremely luminous galaxies.

In 1991 he was first author on one of two papers that convinced the community that our Galaxy is barred. In 1978 he argued that dark halos would be triaxial and investigated the observable consequences of this for disc galaxies. In 1981 he showed that triaxial galaxies have vertical analogues of the Lindblad resonances. In 1982 he and a student demonstrated the importance of velocity anisotropy for measuring black-hole masses. In 1985 he coauthored a paper pointing out the capacity for central black holes to make triaxial galaxies axisymmetric. In 1998 and 1999 his group showed that the “normal mode” theory of galactic warps is not viable and argued that warps are driven by cosmic infall. In 2000 he was coauthor of a paper which argued that published measurements of gravitational microlensing were incompatible with Galactic structure – subsequently observers revised the experimental values down to theoretically acceptable values. From 1990 his group showed how to construct numerically an integrable Hamiltonian that is extremely close to any given galactic Hamiltonian. This technique forms the basis for models of our Galaxy that are currently being developed to extract key science from several large surveys of the Milky Way, culminating in ESA’s cornerstone mission, Gaia. In 2002 he and Sellwood discovered that stars such as the Sun can migrate large distances radially. Recently with a student he has shown that including this effect has a big impact on models of the Galaxy’s chemical evolution. In particular it explains for the first time the metal distribution of stars near the Sun and shows that a “thick disc” of the type observed emerges naturally without the need to an early merger event.